Follicular shuttling of marginal zone B cells facilitates antigen transport

Stromal cell contributions to the homeostasis and functionality of the immune system

A defining characteristic of the immune system is the constant movement of many of its constituent cells through the secondary lymphoid tissues, mainly the spleen and lymph nodes, where crucial interactions that underlie homeostatic regulation, peripheral tolerance and the effective development of adaptive immune responses take place. What has only recently been recognized is the role that non-haematopoietic stromal elements have in many aspects of immune cell migration, activation and survival. In this Review, we summarize our current understanding of lymphoid compartment stromal cells, examine their possible heterogeneity, discuss how these cells contribute to immune homeostasis and the efficient initiation of adaptive immune responses, and highlight how targeting of these elements by some pathogens can influence the host immune response.

Tissue-specific B-cell dysfunction and generalized memory B-cell loss during acute SIV infection

Background

Primary HIV-infected patients display severe and irreversible damage to different blood B-cell subsets which is not restored by highly efficient anti-retroviral therapy (HAART). Because longitudinal investigations of primary HIV-infection is limited by the availability of lymphoid organs, we studied the tissue-specific B-cell dysfunctions in acutely simian immunodeficiency virus (SIV) mac251-infected Cynomolgus macaques.

Methods and Findings

Experiments were performed on three groups of macaques infected for 14, 21 or 28 days and on three groups of animals treated with HAART for two-weeks either initiated at 4 h, 7 or 14 days post-infection (p.i.). We have simultaneously compared changes in B-cell phenotypes and functions and tissue organization of B-cell areas in various lymphoid organs. We showed that SIV induced a steady decline in SIgG-expressing memory (SIgD⁻CD27⁺) B-cells in spleen and lymph nodes during the first 4 weeks of infection, concomitant to selective homing/sequestration of B-cells to the small intestine and spleen. SIV non-specific Ig production was transiently increased before D14p.i., whereas SIV-specific Ig production was only detectable after D14p.i., coinciding with the presence of CD8⁺ T-cells and IgG-expressing plasma cells within germinal centres. Transient B-cell apoptosis on D14p.i. and commitment to terminal differentiation contributed to memory B-cell loss. HAART abrogated B-cell apoptosis, homing to the small intestine and SIV-specific Ig production but had minimal effect on early Ig production, increased B-cell proportions in spleen and loss of memory B-cells. Therefore, virus–B-cell interactions and SIV-induced inflammatory cytokines may differently contribute to early B-cell dysfunction and impaired SIV/HIV-specific antibody response.

Conclusions

These data establish tissue-specific impairments in B-cell trafficking and functions and a generalized and steady memory B-cell loss in secondary lymphoid organs. Characterization of underlying mechanisms would be helpful in designing new therapeutic strategies to dampen B-cell activation and increases HIV/SIV specific antibody response.

B cell acquisition of antigen in vivo

The fate of B lymphocytes is dictated in large part by cognate antigen and the environment in which it is encountered. Yet we are only now beginning to understand where and how B cells acquire antigen. Recent studies identify multiple pathways by which lymph-borne antigens enter the B cell follicles of LNs. Size is a major factor as particulate antigens and large IC are bound by subcapsular sinus macrophages. By contrast, small antigens (under 70kDa) are rapidly channeled into follicles via conduits secreted by fibroblastic reticular cells (FRC). Interestingly, the conduits not only deliver antigen to follicular dendritic cells (FDC) but also provide a rich source of B cell chemokine, that is, CXCL-13. Thus, the follicular conduits provide an 'antigen highway' for B cells trafficking within the LN. These new findings provide an important discovery in understanding how B cells acquire cognate antigen.

The microanatomy of B cell activation

The logistic problem of B cell antigen encounter in the lymph node has recently been studied by dynamic imaging using two-photon microscopy. These studies combined with the early studies of antigen transport have yielded a more complete picture of the orchestration of B cell activation in vivo. Here we summarize the recent advances and focus on the specialized macrophages that are critical to this process and the role of B cells themselves as antigen transporting cells.

Human marginal zone B cells

Human marginal zone (MZ) B cells are, in a sense, a new entity. Although they share many properties with their mouse counterpart, they also display striking differences, such as the capacity to recirculate and the presence of somatic mutations in their B cell receptor. These differences are the reason they are often not considered a separate, rodent-like B cell lineage, but rather are considered IgM memory B cells. We review here our present knowledge concerning this subset and the arguments in favor of the proposition that humans have evolved for their MZ B cell compartment a separate B cell population that develops and diversifies its Ig receptor during ontogeny outside T-dependent or T-independent immune responses.

Role of marginal zone B lymphocytes in invariant NKT cell activation

Splenic marginal zone B (MZB) lymphocytes represent, along with dendritic cells (DC) a first line of defense against blood-borne pathogens. MZB cells express high levels of MHC class II and CD1d molecules but so far their ability to activate and orientate conventional and innate-like T lymphocytes, such as invariant NKT (iNKT) cells, is still elusive. In the present study, we show that murine MZB cells proliferate, mature phenotypically, and secrete cytokines in response to TLR (except TLR3) agonists. When pulsed with OVA peptide (but not whole OVA), MZB cells promote the release of IFN-gamma and IL-4 by Ag-specific CD4(+) T lymphocytes and their stimulation with the TLR9 agonist CpG oligodeoxynucleotide (ODN), a potent MZB cell activator, biases them toward more Th1 inducers. Unlike DC, CpG ODN-stimulated MZB cells fail to stimulate iNKT cells. Although able to activate iNKT hybridomas, MZB cells sensitized with free alpha-galactosylceramide (alpha-GalCer), a CD1d-restricted glycolipid Ag, do not directly activate ex vivo sorted iNKT cells unless DC are added to the culture system. Interestingly, MZB cells amplify the DC-mediated activation of iNKT cells and depletion of MZB cells from total splenocytes strongly reduces iNKT cell activation (cytokine production) in response to alpha-GalCer. Thus, DC and MZB cells provide help to each other to optimize iNKT cell stimulation. Finally, in vivo transfer of alpha-GalCer-loaded MZB cells potently activates iNKT and NK cells. This study confirms and extends the concept that MZB cells are important players in immune responses, a property that might be exploited.

Ectopic germinal centers are rare in Sjogren's syndrome salivary glands and do not exclude autoreactive B cells

This study reports on the characterization of B cells of germinal center (GC)-like structures infiltrating the salivary glands (SGs) of patients with Sjögren's syndrome. Eight two-color combinations were devised to characterize the phenotype of these B cells in 11 SG specimens selected from biopsies obtained from 40 Sjögren's syndrome patients and three normal tonsils. The 9G4 mAb, which recognizes V4.34-encoded autoAbs, enabled us to identify autoreactive B cells. Quantitative RT-PCR was used to determine the level of mRNAs for activation-induced cytidine deaminase (AICDA), repressors and transcription factors. CD20(+)IgD(-)CD38(+)CD21(+)CD24(-) B cells, similar to those identified in tonsil GCs, were seen in the SGs of four patients and, and since they expressed AICDA, they were termed "real GCs". CD20(+)IgD(+)CD38(-)CD21(+)CD24(+) B cells, seen in aggregates from the remaining seven samples, were characteristically type 2 transitional B cells and marginal zone-type B cells. They lacked AICDA mRNAs and were termed "aggregates". Real GCs from SGs contained mRNAs for Pax-5 and Bcl-6, like tonsil GC cells, whereas aggregates contained mRNAs for Notch-2, Blimp-1, IRF-4, and BR3, similar to marginal zone B cells. Further experimental data in support of this dichotomy included the restriction of CXCR5 expression to real GC cells, while sphingosine 1-phosphate receptor 1 was expressed only in aggregates. In contrast, both types of B cell clusters expressed the idiotype recognized by the 9G4 mAb. Our data indicate that, in SGs, a minority of B cell clusters represent genuine GC cells, while the majority manifest features of being type 2 transitional B cells and marginal zone cells. Interestingly, both types of B cell aggregates include autoreactive B cells.

Complement and humoral immunity

The complement system was discovered almost a century ago as an important effector in antibody-dependent killing of microorganisms. Since this early period much was learned aboutthe biochemistry and structure of complement proteins and their function in mediating inflammation. More recently, a prominent role for complement was identified in linkage of innate and adaptive immunity. In this review, I will discuss our current understanding of the importance of complement in enhancing the humoral immune response to both model antigens and pathogens. As discussed below, it is evident that the complement system participates in marking of "foreign" pathogens and "presenting" them to B cells in a manner that enhances both antibody production and long-term memory. In this special issue of Vaccine, we see examples of how complement is critical in the immune response to bacterial and viral pathogens. Moreover, the finding that most organisms have co-evolved proteins to evade complement detection underscores its importance in host protection.

Lipid mediators in health and disease: enzymes and receptors as therapeutic targets for the regulation of immunity and inflammation

Prostaglandins, leukotrienes, platelet-activating factor, lysophosphatidic acid, sphingosine 1-phosphate, and endocannabinoids, collectively referred to as lipid mediators, play pivotal roles in immune regulation and self-defense, and in the maintenance of homeostasis in living systems. They are produced by multistep enzymatic pathways, which are initiated by the de-esterification of membrane phospholipids by phospholipase A2s or sphingo-myelinase. Lipid mediators exert their biological effects by binding to cognate receptors, which are members of the G protein-coupled receptor superfamily. The synthesis of the lipid mediators and subsequent induction of receptor activity is tightly regulated under normal physiological conditions, and enzyme and/or receptor dysfunction can lead to a variety of disease conditions. Thus, the manipulation of lipid mediator signaling, through either enzyme inhibitors or receptor antagonists and agonists, has great potential as a therapeutic approach to disease. In this review, I summarize our current state of knowledge of the synthesis of lipid mediators and the function of their cognate receptors, and discuss the effects of genetic or pharmacological ablation of enzyme or receptor function on various pathophysiological processes.

Two-photon microscopy of host-pathogen interactions: acquiring a dynamic picture of infection in vivo

Two-photon (2P) microscopy has become increasingly popular among immunologists for analysing single-cell dynamics in tissues. Researchers are now taking 2P microscopy beyond the study of model antigen systems (e.g. ovalbumin immunization) and are applying the technique to examine infection in vivo. With the appropriate fluorescent probes, 2P imaging can provide high-resolution spatio-temporal information regarding cell behaviour, monitor cell functions and assess various outcomes of infection, such as host cell apoptosis or pathogen proliferation. Imaging of transgenic and knockout mice can be used to probe molecular mechanisms governing the host response to infection. From the microbe side, imaging genetically engineered mutant strains of a pathogen can test the roles of specific virulence factors in pathogenesis. Here, we discuss recent work that has applied 2P microscopy to study models of infection and highlight the tremendous potential that this approach has for investigating host-pathogen interactions.

Intact bacteria inhibit the induction of humoral immune responses to bacterial-derived and heterologous soluble T cell-dependent antigens

During infections with extracellular bacteria, such as Streptococcus pneumoniae (Pn), the immune system likely encounters bacterial components in soluble form, as well as those associated with the intact bacterium. The potential cross-regulatory effects on humoral immunity in response to these two forms of Ag are unknown. We thus investigated the immunologic consequences of coimmunization with intact Pn and soluble conjugates of Pn-derived proteins and polysaccharides (PS) as a model. Coimmunization of mice with Pn and conjugate resulted in marked inhibition of conjugate-induced PS-specific memory, as well as primary and memory anti-protein Ig responses. Inhibition occurred with unencapsulated Pn, encapsulated Pn expressing different capsular types of PS than that present in the conjugate, and with conjugate containing protein not expressed by Pn, but not with 1-microm latex beads in adjuvant. Inhibition was long-lasting and occurred only during the early phase of the immune response, but it was not associated with tolerance. Pn inhibited the trafficking of conjugate from the splenic marginal zone to the B cell follicle and T cell area, strongly suggesting a potential mechanism for inhibition. These data suggest that during infection, bacterial-associated Ags are the preferential immunogen for antibacterial Ig responses.

Requirement of B cells for generating CD4+ T cell memory

B cells can influence T cell responses by directly presenting Ag or by secreting Ab that binds to Ag to form immunogenic complexes. Conflicting evidence suggests that persisting Ag-Ab complexes propagate long-term T cell memory; yet, other data indicate that memory cells can survive without specific Ag or MHC. In this study, the roles of B cells and Ag-Ab complexes in T cell responses to lymphocytic choriomeningitis virus (LCMV) infection were investigated using B cell-deficient or B cell-competent mice. Despite normal lymphocyte expansion after acute infection, B cell-deficient mice rapidly lost CD4(+) T cell memory, but not CD8(+) T cell memory, during the contraction phase. To determine whether Ag-Ab complexes sustain CD4(+) T cell memory, T cell responses were followed in B cell-transgenic (mIg-Tg) mice that have B cells but neither LCMV-specific Ab nor LCMV-immune complex deposition. In contrast to B cell-deficient mice, mIg-Tg mice retained functional Th cell memory, indicating that B cells selectively preserve CD4(+) T cell memory independently of immune complex formation. An in vivo consequence of losing CD4(+) T cell memory was that B cell-deficient mice were unable to resolve chronic virus infection. These data implicate a B cell function other than Ab production that induces long-term protective immunity.

Conduits mediate transport of low-molecular-weight antigen to lymph node follicles

To track drainage of lymph-borne small and large antigens (Ags) into the peripheral lymph nodes and subsequent encounter by B cells and follicular dendritic cells, we used the approach of multiphoton intravital microscopy. We find a system of conduits that extend into the follicles and mediate delivery of small antigens to cognate B cells and follicular dendritic cells. The follicular conduits provide an efficient and rapid mechanism for delivery of small antigens and chemokines such as CXCL13 to B cells that directly contact the conduits. By contrast, large antigens were bound by subcapsular sinus macrophages and subsequently transferred to follicular B cells as previously reported. In summary, the findings identify a unique pathway for the channeling of small lymph-borne antigens and chemoattractants from the subcapsular sinus directly to the B cell follicles. This pathway could be used for enhancing delivery of vaccines or small molecules for improvement of humoral immunity.

The who, how and where of antigen presentation to B cells

A functional immune system depends on the appropriate activation of lymphocytes following antigen encounter. In this Review, we summarize studies that have used high-resolution imaging approaches to visualize antigen presentation to B cells in secondary lymphoid organs. These studies illustrate that encounters of B cells with antigen in these organs can be facilitated by diffusion of the antigen or by the presentation of antigen by macrophages, dendritic cells and follicular dendritic cells. We describe cell-surface molecules that might be important in mediating antigen presentation to B cells and also highlight the key role of B cells themselves in antigen transport. Data obtained from the studies discussed here highlight the predominance, importance and variety of the cell-mediated processes that are involved in presenting antigen to B cells in vivo.

Visualizing the molecular and cellular events underlying the initiation of B-cell activation

The appropriate activation of B cells is critical for the development of effective immune responses. B cell activation is initiated following the engagement of the B cell receptor (BCR) with specific antigen. The spatiotemporal characterization of the ensuing molecular and cellular events has been the subject of recent high-resolution imaging investigations. In this review we highlight information gathered thus far concerning the initial processes underlying the activation of B cells. First, we consider studies that have offered new insights into the early molecular events that occur within the B cell prior to formation of the immunological synapse. As such, BCR-microclusters formed on engagement with antigen have been identified as the sites of active signaling and assembly of "microsignalosomes." Furthermore, signaling through these "microsignalosomes" is propagated and enhanced through B cell spreading in response to membrane-antigen in a CD19-dependent manner. Finally, we discuss a number of multiphoton microscopy studies that have enabled dynamic characterization of the initial encounters between B cells and antigen in vivo. These investigations visualize the presentation of larger antigens to B cells via cell-mediated strategies, involving macrophages in the subcapsular sinus and dendritic cells in the paracortex.

The alliance of sphingosine-1-phosphate and its receptors in immunity

Sphingosine-1-phosphate (S1P) is a biologically active metabolite of plasma-membrane sphingolipids that is essential for immune-cell trafficking. Its concentration is increased in many inflammatory conditions, such as asthma and autoimmunity. Much of the immune function of S1P results from the engagement of a family of G-protein-coupled receptors (S1PR1-S1PR5). Recent findings on the role of S1P in immunosurveillance, the discovery of regulatory mechanisms in S1P-mediated immune-cell trafficking and new advances in understanding the mechanism by which S1P affects immune-cell function indicate that the alliance between S1P and its receptors has a fundamental role in immunity.

Bacterial entry to the splenic white pulp initiates antigen presentation to CD8+ T cells

The spleen plays an important role in host-protective responses to bacteria. However, the cellular dynamics that lead to pathogen-specific immunity remain poorly understood. Here we examined Listeria monocytogenes (Lm) infection in the mouse spleen via in situ fluorescence microscopy. We found that the redistribution of Lm from the marginal zone (MZ) to the periarteriolar lymphoid sheath (PALS) was inhibited by pertussis toxin and required the presence of CD11c(+) cells. As early as 9 hr after infection, we detected infected dendritic cells in the peripheral regions of the PALS and clustering of Lm-specific T cells by two-photon microscopy. Pertussis toxin inhibited both Lm entry into the PALS and antigen presentation to CD8(+) T cells. Our study suggests that splenic dendritic cells rapidly deliver intracellular bacteria to the T cell areas of the white pulp to initiate CD8(+) T cell responses.

WASP confers selective advantage for specific hematopoietic cell populations and serves a unique role in marginal zone B-cell homeostasis and function

Development of hematopoietic cells depends on a dynamic actin cytoskeleton. Here we demonstrate that expression of the cytoskeletal regulator WASP, mutated in the Wiskott-Aldrich syndrome, provides selective advantage for the development of naturally occurring regulatory T cells, natural killer T cells, CD4(+) and CD8(+) T lymphocytes, marginal zone (MZ) B cells, MZ macrophages, and platelets. To define the relative contribution of MZ B cells and MZ macrophages for MZ development, we generated wild-type and WASP-deficient bone marrow chimeric mice, with full restoration of the MZ. However, even in the presence of MZ macrophages, only 10% of MZ B cells were of WASP-deficient origin. We show that WASP-deficient MZ B cells hyperproliferate in vivo and fail to respond to sphingosine-1-phosphate, a crucial chemoattractant for MZ B-cell positioning. Abnormalities of the MZ compartment in WASP(-/-) mice lead to aberrant uptake of Staphylococcus aureus and to a reduced immune response to TNP-Ficoll. Moreover, WASP-deficient mice have increased levels of "natural" IgM antibodies. Our findings reveal that WASP regulates both development and function of hematopoietic cells. We demonstrate that WASP deficiency leads to an aberrant MZ that may affect responses to blood-borne pathogens and peripheral B-cell tolerance.

Intrafollicular location of marginal zone/CD1d(hi) B cells is associated with autoimmune pathology in a mouse model of lupus

Marginal zone (MZ) B cells contain a large number of autoreactive clones and the expansion of this compartment has been associated with autoimmunity. MZ B cells also efficiently transport blood-borne antigen to the follicles where they activate T cells and differentiate into plasma cells. Using the B6.NZM2410.Sle1.Sle2.Sle3 (B6.TC) model of lupus, we show that the IgM+ CD1d(hi)/MZ B-cell compartment is expanded, and a large number of them reside inside the follicles. Contrary to the peripheral B-cell subset distribution and their activation status, the intrafollicular location of B6.TC IgM+ CD1d(hi)/MZ B cells depends on both bone marrow- and stromal-derived factors. Among the factors responsible for this intrafollicular location, we have identified an increased response to CXCL13 by B6.TC MZ B cells and a decreased expression of VCAM-1 on stromal cells in the B6.TC MZ. However, the reduced number of MZ macrophages observed in B6.TC MZs was independent of the IgM+ CD1d(hi)/B-cell location. B7-2 but not B7-1 deficiency restored IgM+ CD1d(hi)/MZ B-cell follicular exclusion in B6.TC mice, and it correlated with tolerance to dsDNA and a significant reduction of autoimmune pathology. These results suggest that follicular exclusion of IgM+ CD1d(hi)/MZ B cells is an important B-cell tolerance mechanism, and that B7-2 signaling is involved in breaching this tolerance checkpoint.

T cell-independent development and induction of somatic hypermutation in human IgM+ IgD+ CD27+ B cells

IgM⁺IgD⁺CD27⁺ B cells from peripheral blood have been described as circulating marginal zone B cells. It is still unknown when and where these cells develop. These IgM⁺IgD⁺CD27⁺ B cells exhibit somatic hypermutations (SHMs) in their B cell receptors, but the exact nature of the signals leading to induction of these SHMs remains elusive. Here, we show that IgM⁺IgD⁺CD27⁺ B cells carrying SHMs are observed during human fetal development. To examine the role of T cells in human IgM⁺IgD⁺CD27⁺ B cell development we used an in vivo model in which Rag2^−/−γ_C^−/− mice were repopulated with human hematopoietic stem cells. Using Rag2^−/−γ_C^−/− mice on a Nude background, we demonstrated that development and induction of SHMs of human IgM⁺IgD⁺CD27⁺ B cells can occur in a T cell–independent manner.

The vascular S1P gradient-cellular sources and biological significance

Sphingosine 1-phosphate (S1P), a product of sphingomyelin metabolism, is enriched in the circulatory system whereas it is estimated to be much lower in interstitial fluids of tissues. This concentration gradient, termed the vascular S1P gradient appears to form as a result of substrate availability and the action of metabolic enzymes. S1P levels in blood and lymph are estimated to be in the muM range. In the immune system, the S1P gradient is needed as a spatial cue for lymphocyte and hematopoietic cell trafficking. During inflammatory reactions in which enhanced vascular permeability occurs, a burst of S1P becomes available to its receptors in the extravascular compartment, which likely contributes to the tissue reactions. Thus, the presence of the vascular S1P gradient is thought to contribute to physiological and pathological conditions. From an evolutionary perspective, S1P receptors may have co-evolved with the advent of a closed vascular system and the trafficking paradigms for hematopoietic cells to navigate in and out of the vascular system.

Regulation of thymus-dependent and thymus-independent production of immunoglobulin G subclasses by Galpha12 and Galpha13

Background

A previous study from this laboratory showed that Gα₁₂ members participate in the production of inflammatory cytokines. In spite of the identification of B cell homeostasis responses regulated by Gα₁₃, the functional roles of Gα₁₂ members in the production of immunoglobulin (Ig) isotypes remained unknown. This study investigated whether Gα₁₂ members are involved in the Ig isotype antibody production with the purpose of establishing their functions in thymus-dependent and thymus-independent humoral responses.

Results

Mice lacking Gα₁₂ and/or Gα₁₃ showed an impaired antigen-specific antibody production promoted by challenge(s) of ovalbumin or trinitrophenyl-lipopolysaccharide (TNP-LPS), used for thymus-dependent and thymus-independent stimuli, respectively. Homozygous knockout (KO) of Gα₁₂ or double heterozygous KO of Gα₁₂/Gα₁₃ significantly reduced the antigen-specific total IgG level after multiple ovalbumin immunizations with decreases in the production of IgG1, IgG2a and IgG2b subclasses, as compared to wild type control. In contrast, IgM production was not decreased. Moreover, mice deficient in Gα₁₂ or partially deficient in Gα₁₃ or Gα₁₂/Gα₁₃ showed significantly low production of IgG2b in response to TNP-LPS. In TNP-LPS-injected mice, IgG1 and IgG2a productions were unaffected by the G protein KOs.

Conclusion

Our results demonstrate that both Gα₁₂ and Gα₁₃ are essentially involved in thymus-dependent and independent production of IgG subclasses, implying that the G-proteins contribute to the process of antigen-specific IgG antibody production.

New insights into the early molecular events underlying B cell activation

The appropriate activation of B cells is critical for the development and operation of immune responses and is dependent on the extensive coordination of intra- and intercellular communications in response to antigen stimulation. An accurate description of the B cell-activation process requires investigation of these interactions within their correct cellular context both at high resolution and in real time. Here, we discuss a number of recent studies that have offered insight into the early molecular events of B cell activation. We suggest that segregation within the B cell membrane triggers localized cytoskeleton reorganisation and signaling, allowing the formation of B cell receptor (BCR) microclusters. These BCR microclusters are the sites for the coordinated recruitment of the signalosome and are propagated during B cell spreading. We discuss the recent identification of a critical role for CD19 in the B cell response to membrane-bound antigen and suggest a mechanism involving BCR microclusters by which it mediates its stimulatory function. Finally, we consider research that has taken advantage of recent technological advances in multiphoton microscopy that have allowed its application to the investigation of the dynamics of membrane-bound antigen presentation and subsequent B cell activation in lymph nodes in vivo.

Imaging techniques: new insights into chemokine/chemokine receptor biology at the immune system

Our current knowledge of molecular and cellular responses in vivo is based mainly on event reconstruction from time-freeze observations. Conventional biochemical and genetic methods consider the cell as an individual entity and ligand/receptor pairs as isolated systems. In addition, the data refer to the average behavior of a pool of cells and/or receptors removed from their real-life context. The use of new technologies, particularly real-time imaging approaches, is showing us that biological responses are highly dynamic and extremely dependent on the context in which they take place, and therefore much more diverse than initially envisaged. This review focuses on the mechanistic insights that new imaging techniques, such as those based on resonance energy transfer and two-photon microscopy, contribute to our understanding of how receptors work within a single cell, and how cells work within a tissue. Cell movement is a complex and regulated process; it has a key role in embryogenesis, organogenesis, wound-healing and tumor invasion. Nonetheless, it is in immune system homeostasis and response that cell movement becomes essential. For this reason, immunology is being radically transformed and enriched by these new approaches. We will discuss the use of these techniques for studying chemokine/chemokine receptors and their role in the immune system function, and comment on the potential contribution to the design of therapeutic strategies.

DNA microarray gene expression profile of marginal zone versus follicular B cells and idiotype positive marginal zone B cells before and after immunization with Streptococcus pneumoniae

Marginal zone (MZ) B cells play an important role in the clearance of blood-borne bacterial infections via rapid T-independent IgM responses. We have previously demonstrated that MZ B cells respond rapidly and robustly to bacterial particulates. To determine the MZ-specific genes that are expressed to allow for this response, MZ and follicular (FO) B cells were sort purified and analyzed via DNA microarray analysis. We identified 181 genes that were significantly different between the two B cell populations. Ninety-nine genes were more highly expressed in MZ B cells while 82 genes were more highly expressed in FO B cells. To further understand the molecular mechanisms by which MZ B cells respond so rapidly to bacterial challenge, Id-positive and -negative MZ B cells were sort purified before (0 h) or after (1 h) i.v. immunization with heat-killed Streptococcus pneumoniae, R36A, and analyzed via DNA microarray analysis. We identified genes specifically up-regulated or down-regulated at 1 h following immunization in the Id-positive MZ B cells. These results give insight into the gene expression pattern in resting MZ vs FO B cells and the specific regulation of gene expression in Ag-specific MZ B cells following interaction with Ag.

Peripheral B cell subsets

Our understanding of the origins and the biological functions of different peripheral B cell subsets continues to evolve. Some understanding has been obtained regarding the synergy between BCR-derived signals and other receptors and signaling pathways that drive the development of follicular, marginal zone, and B-1 B cells, but this remains a complex and poorly understood issue. More recent information regarding the origins of B-1 and B-2 B cells, the ability of follicular B cells to mature both in the bone marrow and the spleen, the existence of a definable precursor for MZ B cells, and the ability of follicular B cells to occupy two distinct niches are all highlighted in this review.

Geography and plumbing control the T cell response to infection

The orchestrated movement of cells of the immune system is essential to generation of productive responses leading to protective memory development. Recent advances have allowed the direct microscopic visualization of lymphocyte and antigen-presenting cell migration and interaction during immune response initiation and progression. These studies have defined important characteristics of the microanatomy of lymphocyte movement, particularly in the lymph node. Moreover, the ability to track endogenous antigen-specific T cells has revealed a coordinated pathway of CD8 T cell movement in the spleen following primary and secondary infection. As a consequence, the local anatomy of secondary lymphoid tissues during infection has emerged as a critical regulator of immunity. While some of the factors responsible for the migratory cues instructing immune cell movement have been identified, much remains to be learned. Here, we provide a brief overview of studies examining CD8 T cell localization during the immune response to infection in the context of our current understanding of immune system structure.